This is one of Aesop's lesser-known tales, especially in the form below in which I've made my own rough (and a bit extended) translation. But, as is the case with all of Aesop's fine fables, this one has much to remind us of common sense, morality and effective living.
There was once in the Greek countryside a large, bald man who sat down under a tree after a hard day's labor at his kiln. A fly came up and kept buzzing about his plain pate, making the man's summer respite a bitter ordeal, particularly as the fly's irritating buzz was accompanied by bites on the man's unprotected head.
In the first minutes of his consternation, the bald man aimed blow after infuriated blow at his tiny enemy, but his only accomplishment was in whacking himself on his unprotected noggin.
Finally, the man's self-inflicted pain brought him to his senses. And when the fly swooped down again to torment him, the now-wiser man bore his suffering with greater forbearance as he said to himself, "A man will only injure himself if he takes notice of despicable enemies."